Multiple Alleles and Lethals 



65 



these populations is completely dominant to 

 ci. But the hybrid of ci with wild flies from 

 population 3, c/+^ ci, shows the cubitus vein 

 interrupted! Moreover, the lack of domi- 

 nance of c/+'^ over ci can be shown to be an 

 effect of this gene pair rather than a modify- 

 ing effect of some other gene pair. Appar- 

 ently, then, the c/+ allele in population 3 is 

 different from the one in populations 1 and 2. 



Thus, alleles which at first seem alike may 

 prove to be different when tested further. 

 Such alleles are said to be isoalleles. Some 

 other techniques which may be employed to 

 detect isoalleles include the response of the 

 tested alleles to the presence of nonallelic 

 genes, to environmental changes as of tem- 

 perature and humidity, and to agents which 

 modify mutation rates. The number of alleles 

 that can be proven isoallelic will depend upon 

 how many different phenotypic criteria you 

 employ to compare alleles, and how small a 

 phenotypic difference you are able to recog- 

 nize. The more delicate the tests and the 

 larger their number, the greater is the chance 

 for demonstrating isoallelism. 



Although we have described isoalleles 

 among the genes normally expressed in indi- 

 viduals living in the wild (wild type isoalleles), 

 there are also isoalleles for mutant genes 

 (mutant isoalleles). For instance, it has been 

 proven that the mutant gene w, producing 

 white eye in different strains of Drosophila, 

 actually comprises a series of multiple iso- 

 alleles {w\ w~, w^, etc.). 



Lethals 



In the snapdragon (Antirrhinum) one can find 

 two kinds of full grown plants, those which 

 are green and those which are a paler green 

 called auria. Green crossed by green pro- 

 duces only green, but auria by auria produces 

 seedhngs of which 25% are green (AA), 

 50% auria (Aa), and 25% white (aa). The 

 latter die, after exhausting the food in the 

 seed from which they grew, because they lack 

 chlorophyll and cannot synthesize food. So, 



among the grown plants, the phenotypic ratio 

 observed is % green : % auria. In this case, 

 then, lack of dominance gives a 1:2:1 

 ratio among seedlings, characteristic of a 

 cross between monohybrids, which because 

 of lethality becomes a 2 : 1 ratio among the 

 older survivors. 



These ratios were discovered in the reverse 

 order in a case in mice. In this case, in which 

 genetic lethality was first demonstrated, 

 crosses between two yellow mice never gave 

 all yellow progeny, but always gave 2 yel- 

 low : 1 nonyellow. It was then shown that 

 from such matings % of the fertilized eggs 

 which should have completed development 

 failed to do so and aborted early. Those dy- 

 ing were clearly the homozygotes of one type, 

 with nonyellows being the other homozygous 

 type, since crosses between nonyellows pro- 

 duced only nonyellows. Note that the gene 

 symbols usually employed will not be satis- 

 factory here. For now we have two effects 

 to describe for each gene — one effect on 

 color and one on viabihty. Moreover, the 

 allele which is dominant for the one effect is 

 recessive for the other, and vice versa. This 

 problem is solved by using base letters with 

 superscripts for each gene (Figure 9-3), 

 where the base letter refers to one trait and 

 the superscript refers to the other trait. Let 

 the superscript / be the recessive lethal eff"ect 

 of the gene dominant for yellow, Y, and L 

 be the superscript for the dominant normal 

 viability of the allele recessive for nonyellow, 

 y. Then the Fi from crossing two yellow mice 

 (ry^ X Y'y^) are I FT' (dies) : 2 Y'y^ 

 (yellow) : 1 y^y^ (nonyellow). 



In both the snapdragon and mouse cases 

 described, death resulted from the presence 

 of an allele in homozygous condition. Those 

 alleles which kill the individual before it can 

 reproduce are called lethal genes or lethals 

 — those producing this effect only when 

 homozygous are recessive lethals, while those 

 acting this way when heterozygous are domi- 

 nant lethals. 



